1. Field of the Invention
The present invention relates to leadframe materials for use in semiconductor devices. More particularly, it relates to copper alloys for leadframes having excellent electric conductivity and mechanical strength.
2. Description of the Prior Art
Semiconductor chips having integrated circuits are usually connected to leadframes by means of wires, and they are packaged in molded resins except for leadframe terminals, whereby IC devices are provided. Leadframe materials which may be used for such integrated circuit devices are, in general, required to have the following properties:
(1) Good electric and thermal conductivities
Since leadframes serve as electric conductors for supplying electric signals to the circuits, they should have good electric conductivity. Also, since heat generated in the circuits should be dissipated through the leadframes, they should have good thermal conductivity, too. In general, thermal conductivity is proportional to electric conductivity.
(2) High mechanical strength
When the leadframe terminals of semiconductor devices are inserted into sockets of circuit boards, misalignments of the terminals and the sockets may occur. Accordingly, the leadframe terminals should be tough enough not to be bent when pressed under misaligned conditions. Further, they should have high resistance to fatigue by repeated bending.
(3) Good heat resistance (high softening point)
During the production of semiconductor devices, die bonding, wire bonding and resin molding are performed, exposing leadframes to high temperatures of 300.degree.-450.degree. C. If the leadframes are largely softened by exposure to such temperatures, they would be deformed even by small force at room temperature. This is detrimental to the leadframes. Accordingly, leadframes must have good heat resistance, namely high softening temperature so that they have enough resistance to mechanical deformation at room temperature.
(4) Coefficient of thermal expansion close to those of semiconductor chips or molded resin packages
If there is large difference in coefficient of thermal expansion between leadframes and semiconductor chips or molded resin packages, distortions would occur during the assembling steps involving heating, due to the difference in their thermal expansion coefficients. Such distortions might cause the variations of semiconductor chips' characteristics and the deteriorations of the adhesion of the leadframes to resin packages. In order to prevent this, the leadframes should have thermal expansion coefficients close to those of the semiconductor chips or the molded resin packages.
(5) Good platability
Leadframes are plated with gold or silver in portions which are subjected to die bonding with the semiconductor chips. Therefore, leadframes should have good affinity to platings. That is, platings should be strongly adhered to the leadframe surfaces and should have as small defects as possible.
(6) Good solderability
Leadframe terminals are soldered before or after mounting thereof on circuit boards. Accordingly, leadframes should have good solderability, namely they should be highly wettable with solders.
(7) Good solder durability
Semiconductor devices soldered on circuit boards should not deteriorate their characteristics during their entire life. In general, soldered portions are one of those vulnerable to deterioration. Therefore, the soldered portions of the leadframes should be able to withstand any possible environment in which semiconductor devices are used, without deteriorating the adhesion thereof to solder layers. Such property is called herein "solder durability."
(8) Good adhesion to molded resin packages
In general, most semiconductor integrated circuit devices are packaged with molded resins. Accordingly, leadframes are required to have good adhesion to molded resins.
Typical alloys for leadframes are iron-nickel alloys and copper-base alloys. As iron-nickel alloys, Fe-42% Ni alloys and Fe-29% Ni-17% Co alloys are known. These alloys have excellent mechanical strength, but their electric conductivity is not always satisfactory. On the other hand, copper-base alloys have good electric conductivity, and are much less expensive than the iron-nickel counterparts. Particularly from the economic point of view, the copper-base alloys have recently been finding rapidly increasing use in leadframes.
U.S. Pat. No. 4,249,941 to R. Futatsuka, et al. discloses copper-base alloys for leadframes of integrated circuit devices consisting essentially of 0.5-1.5 weight % Fe, 0.5-1.5 weight % Sn, 0.01-0.35 weight % P and balance Cu and inevitable impurities. They are, however, not necessarily satisfactory in terms of mechanical strength and solder durability.
U.S. Pat. No. 4,337,089 to K. Arita, et al. discloses copper-nickel-tin alloys for leadframes containing 0.5-3.0 weight % Ni, 0.3-0.9 weight % Sn, 0.01-0.2 weight % P, 0-0.35 weight % Mn and/or Si, and balance Cu. These alloys have good tensile strength but relatively poor electric conductivity. In addition, they do not have sufficient solder durability.
There are some other copper-base alloys: Copper Alloy C 194 (2.35% Fe-0.03% P-0.12% Zn), Copper Alloy C 195 (1.5% Fe-0.1% P-0.8% Co-0.6% Sn), Copper Alloy C 155 (0.06% P-0.07% Ag-0.11% Mg) and Copper Alloy C 151 (0.1% Zr). See "Leadframe Materials for Packaging Semiconductors," SEMICONDUCTOR INTERNATIONAL, September 1982, pp. 111-124. These copper-base alloys, however, do not necessarily have a preferable combination of good electric conductivity and high mechanical strength. Further, they are not satisfactory in terms of solder durability.